1. Field of the Invention
The present invention relates to the field of gas turbines, such as those fitted to the turbine engines of aircraft. The invention relates more particularly to the compression stage of such a gas turbine.
2. Description of the Related Art
A gas turbine compression stage generally comprises a rotary compressor designed to compress the fresh air entering the gas turbine. Amongst the various known types of compressor, it is possible to distinguish between compressors of centrifugal type, and compressors of axial type. Unlike an axial compressor, a centrifugal compressor changes the flow direction of the air stream: on leaving the compressor, the air stream generally flows in a direction that is substantially orthogonal to the axis of rotation of the compressor, and in particular an angle of inclination in the range 0° to 30° is acceptable between the air stream (i.e. the stream of air at the trailing edge of the impeller) and a direction orthogonal to the axis of rotation of the compressor.
More precisely, the present invention relates to a compressor impeller having at least one blade connected to an impeller hub via a fillet or junction of curved shape, said blade extending along a chord defined between a leading edge and a trailing edge of the blade, the junction presenting a shape that varies continuously along the chord.
Conventionally, the air stream entering into the impeller of a centrifugal compressor comes initially into contact with the leading edge of the blade and then runs along the chord of the blade prior to leaving the impeller via the trailing edge.
In the meaning of the present invention, the junction is the geometrical line connecting the blade to the hub at the location of the blade root, which root defines the connection between the blade and the hub. There is a connection on each side of the blade, on the pressure side and on the suction side.
In operation, the blades of the compressor impeller are subjected to high levels of mechanical stress that, under certain circumstances, are capable of damaging them (crack initiation, crack propagation, loss of a fragment of the blade).
The use of a junction that is curved in shape then makes it possible, under certain circumstances and depending on how the selected curved shape is defined, to improve or degrade the mechanical strength of blades, in particular in the blade root zone.
Depending on the zone of the blade (defined along its chord), it is possible either to improve the mechanical strength of the blade or to reduce the mechanical strength of the blade. This applies in particular at the trailing edge of the impeller. The consequence of degrading the mechanical strength of the blade at the trailing edge of the impeller is to provide an improvement relating to the mechanical strength of the disk in the zone of the trailing edge. This may be desirable in order to comply with regulations. Regulations recommend a criterion for rupture ordering in which the blade must rupture before the disk (hub). In other words, the blade acts as a fuse. Given the centrifugal force and the inertia of the hub, a rupture in the hub would generate debris with greater kinetic energy than that generated by debris from a rupture of a blade. The trailing edge of the impeller is particularly concerned by this criterion insofar as the thicknesses of the blade and of the hub (web) are of the same order of magnitude in that zone.
DE 10 16 888 shows a junction between a blade and a hub that varies along the chord.